1. Field of the Invention
The present invention relates to an optical communications system, and more particularly, to a multi-path interference light measuring method and apparatus which input rectangular-wave-modulated light to a target to be measured, such as an optical amplifier, and measure multi-path interference light on an output side, for example, like a pulse-OSA (Optical Spectrum Analyzer) method.
2. Description of the Related Art
In recent years, the speed and the capacity of an optical communication have been increasing with technology such as wavelength multiplexing, etc. Additionally, a rare-earth-doped fiber optical amplifier using an erbium-doped fiber, etc., and an optical amplifier using Raman effect have been advanced, and a linear repeater that amplifies light as it is has been put into practical use.
In a communications system using such optical amplifiers, one of problems which can possibly be a fault is noise, namely, a degradation of S/N ratio (signal to noise ratio). The first cause of the degradation of S/N ratio is spontaneous scattering light, namely, amplified spontaneous emission (ASE) light of an optical amplifier. The second cause of the degradation of the S/N ratio is multi-path interference light noise caused by double Rayleigh scattering (DRS) light of signal light, or reflection at the end of a connector.
An electric spectrum analyzer method, a pulse-OSA (Optical Spectrum Analyzer) method, etc. are used as methods measuring such noise. However, the pulse-OSA method is considered to be effective as a method measuring multiple-path interference light.
The following two documents exist as documents for measuring noise by using the pulse-OSA method.
Document 1) Japanese Patent Publication No. 08(1996)-114528, “Optical Amplifier Noise Figure Measuring Method and Apparatus”.
Document 2) S. A. E. Lewis. et al., “Characterization of Double Rayleigh Scatter Noise in Raman Amplifiers”, IEEE Photonic Technology Letters, Vol. 12, No. 5, pp. 528–530 (2000).
Document 1 discloses an optical amplifier noise figure measuring method and apparatus that can easily adjust the phases of the whole of a system to be measured including an optical fiber in a pulse-OSA method as a method measuring the noise figure of an optical fiber amplifier, especially, an optical fiber amplifier using a rare-earth-doped fiber such as an erbium-doped fiber, etc.
Document 2 proposes a measurement method using a pulse-OSA method in order to measure the noise light of a Raman amplifier that is significantly influenced by multi-path interference light of signal light in addition to spontaneous Raman scattering light. Unlike the spontaneous Raman scattering light, the multi-path interference light is a noise light component which occurs only on a signal light wavelength, and cannot be measured with an interpolation method or a probe method. Therefore, in Document 2, measurement using the pulse-OSA method is made by assuming that a pulse frequency is 500 kHz, the duty ratio of a pulse signal on an input side is 0.1, and the duty ratio of a modulation pulse for output signal light is 0.5.
With the pulse-OSA method, light emitted from a light source is modulated generally with a rectangular pulse whose cycle is sufficiently shorter than the lifetime of an atom having a high energy level, for example, of a rare-earth-doped fiber, and an optical pulse signal after being modulated is input to an optical amplifier to be measured. Then, an output pulse signal is modulated by using a rectangular pulse of an opposite phase, which has the same cycle as the pulse signal output from the optical amplifier to be measured, a noise light component is extracted, and the wavelength dependency of the power of the noise light is observed, for example, by combining a splitter and a plurality of photoreceivers, whereby the noise figure of ASE noise light of the amplifier to be measured can be measured.
If the ASE noise of an erbium-doped fiber amplifier is measured by using a pulse-OSA method as described above, a pulse cycle of approximately 1/100 or shorter of a transition time of several to several tens of milliseconds is used. This is because the lifetime of a spontaneous emission atom is relatively long.
However, the response time of multi-path interference light significantly varies depending on the length of an amplification medium. In document 2, the pulse frequency is held constant to be 500 kHz. However, if a pulse frequency is made constant in this way, multi-path interference light cannot be measured with high accuracy depending on a condition such as the length of an optical amplification medium, or the like.